Method for removing excess coating material from strip edges

ABSTRACT

A method for removing excess coating material from a coated strip in a slow speed coating operation in which the coating weight control is effected by an air knife. There is a tendency for the excess material to form &#39;&#39;&#39;&#39;whiskers&#39;&#39;&#39;&#39; at the strip edges. A wiper is positioned below the air knife and maintained in bearing relation against the strip edge to continuously scrape the excess material from the strip edge before it forms whiskers.

United States Patent 1 1 1 1 3,881,040

Patterson Apr. 29, 1975 METHOD FOR REMOVING EXCESS [56] References CitedCOATING MATERIAL FROM STRIP EDGES UNITED STATES PATENTS [75] Invent r:Rob t Patt n. ai fi 3,480.46) ll/l969 Shaffer 117/102 M [73] Assignee:United States Steel Corporation.

Pi b h P Primary ExaminerMayer Weinblatt Assistant E.\'aminerEdith L.Rollins [22] Filed: Apr. I2, 1974 211 Appl. No.1 460,320 [57] ABSTRACTRelated Application Data A method for removing excess coating materialfrom a [60] Continuation-impart of Scr. No. 247.627. April 26. coatedstrip in a slow speed coating operation in 1972. bandoned. which is adivision of Ser. No. which the coating weight control is effected by anair 8 3. 1971. PM 1 9 knife. There is a tendency for the excess materialto form whiskers" at the strip edges. A wiper is posi- Cl /357 tionedbelow the air knife and maintained in bearing [5 l] Int. Cl..... B05c11/10 relation against the strip edge to continuously scrape Field ofseal'dl o l /1 102 102 the excess material from the strip edge before itforms ll7/ll3. 114 A, US; 118/63 whiskers.

4 Claims, 2 Drawing Figures 1 METHOD FOR REMOVING EXCESS COATINGMATERIAL FROM STRIP EDGES This application is a continuation-in-part ofapplication Ser. No. 247,627 filed Apr. 26, 1972, now abandoned which isa division of application Ser. No. 117,987 filed Feb. 23, 1971, now US.Pat. No. 3,685m484.

This invention relates to a method for removing excess coating materialfrom the edges of coated strip produced in a slow speed coatingoperation in conjunction with air knives which are used for coatingweight control.

Air knives are used to control the weight and distribution of a coatingapplied in hot dip coating operations, such as the galvanizing of steelstrip. Air of a fluid under pressure is directed agaist the face of acoated substrate as it emerges vertically from a bath of molten coatingmaterial. This acts as a squeegee, removing excess coating material andcausing it to flow back down the strip into the bath. The air jets maybe horizontal but are generally inclined downwardly from 5 to 45 fromthe horizontal with about 20 to 25 being preferred for slow speed lines.Air knives, such as shown in my US. Pat. No. 3,406,656, tend to avoidbuild-up of the coating metal at the edges of the substrate when thesubstrate travels at relatively high speeds. However, when hot dipcoating lines, such as a minimum spangle galvanizing line, are operatedat speeds less than 150 feet per minute and air knives are used forcoating control, whiskers are formed which adhere to each edge of thesubstrate. This is a particularly undesirable condition. Besidespresenting a very unappealing appearance, whiskers cause safety andhandling problems and cause coiling difficulties. Further, the cost ofproduction is increased by loss of coating material as whiskers.

It is primary object of my invention to preventthe formation ofdendritic structures or whiskers at the edges of coated substrates whenair knives are used to control the coating weight in a slow speeddip-coating process.

I have found that by mechanically wiping the edge of a slow speedgalvanizing line at the proper location, the formation of whiskers isavoided.

One embodiment of my invention is shown in the attached figures inwhich:

FIG. 1 is a diagrammatic elevation view of a hot dip coating apparatuswith parts broken away.

FIG. 2 is an isometric view of a coated steel strip passing between airknives and mechanical edge wipers.

As shown in FIG. I, a strip enters a vessel 11 containing coatingmaterial 12 which in the galvanizing process is molten zinc, passesaround a sinker roll 13 and moves upwardly between a pair ofoppositelydisposed air knives l4 and is being contacted at its edges bywipers 16. The portion of the wiper in contact with the strip edge isA-shaped (FIG. 2). The cross bar 17 of the A actually contacts the stripwhile the legs 18 prevent the wiper from slipping off the strip edge.The wiper has an extended handle 20 which is adjustably positioned intubular holder 21 and is retained therein by set screw 22. Affixed toholder 21 is an arm 24 which is carried by a support rod or axle 25having integral retaining discs 26 at its ends. The edge wiper ispositioned by placing the support rod 25 in a pair of toothed racks 27,only one off which is shown. A counterweight 28 is hung on a notch 29 inthe arm to maintain sufficient bearing pressure of the wiper against theedge of the strip. I r

' Unexpectedly, Iv found that the wiper is ineffective unless itcontacts the strip at a point between about 1/2 inch and 4 inches',below the point of impact of the air jet from the air knife. .Theoptimum distance has proven to be about one inch below the point ofimpact of the air jet. The wiping must be accomplished while the zinc isstill in the molten state and not after it becomes solidified. If thewiper is closer than l/2 inch from the air jet, the air is turbulent andinterferes with the wiping action. Also, the whiskers may already haveformed and solidified. The following table shows the condition of thestrip edge after cooling of the coating when the edge wiper is placed atvarious distances beneath the point of impact of the air jet against thestrip.

TABLE I Distance Beneath Jet Impact Strip Edge Condition UnacceptableHeavy whiskers A" Unacceptable Light whiskers b" Acceptable 1" Excellent3" Very good 4" Acceptable for sheet Unacceptable for coils 5"Unacceptable Same as without wiper When the wiper is too far beneath theair jet, the molten zinc has not had time to move downward in relationto the strip, and the zinc climbs over the wiper, re-

adhering to the strip edge. Further, the molten zinc tically by use ofset screw 22 and horizontally to accommodate various widths of strip byrepositioning the support rod 25 in the toothed rack 27.

Edge wipers for removing excess coating material from strip edges areknown. However, when an air knife is used, the vertical distance inwhich the wiper must operate is confined to about 4 inches, that beingthe distance between the bath and the air knife. Previous wipers, suchas shown in LeCroy US. Pat. No. 3,536,038 do not have the necessarycompactness to operate in this location. Shaffer US. Pat No. 3,480,469shows a strip guide for use with an air knife, but his guide is not awiper, as it is used to maintain a vacuum aspirator or wiper against thesrrip edge.

It can readily be seen from the foregoing that I have invented a methodfor preventing the formation of whiskers in slow speed hot dip coatinglines in which the coating weight is controlled by use of an air knife.

I claim:

1. In a hot dip process for coating a strip by passing said strip, at aspeed less than ft./min., through a bath of coating material, removingsaid strip vertically from said bath, and controlling the coatingthickness and distribution by directing a jet of fluid from a fluidnozzle against the face of the vertically-moving coated strip, theinclination of said jet being variable from normal to said strip to 45downward, the improvementcomprising preventing the formation of whiskersof coating material on the strip edge by contacting the edges of thestrip by a mechanical wiper at a point bepoint of contact of said wiperis about 1 inch beneath said point of impact of said fluid jet.

3. An improvement according to claim 1 in which said strip is steel.

4. An improvement according to claim 1 in which said coating material iszinc.

1. IN A HOT DIP PROCESS FOR COATING A STRIP BY PASSING SAID STRIP, AT ASPEED LESS THAN 150 FT./MIN., THROUGH A BATH OF COATING MATERIAL,REMOVING SAID STRIP VERTICALLY FROM SAID BATH, AND CONTROLLING THECOATING THICKNESS AND DISTRIBUTION BY DIRECTING A JET OF FLUID FROM AFLUID NOZZLE AGAINST THE FACE OF THE VERTICALLY-MOVING COATED STRIP, THEINCLINATION OF SAID JET BEING VERIABLE FROM NORMAL TO SAID STRIP TO 45*DOWNWARD, THE IMPROVEMENT COMPRISING PREVENTING THE FORMATION OFWHISKERS OF COATING MATERIAL ON THE STRIP EDGE BY CONTACTING THE EDGESOF THE STRIP BY A MECHANICAL WIPER AT A POINT BETWEEN 1/2 AND 4 INCHESBENEATH THE POINT OF IMPACT OF THE FLUID JET AGAINST SAID STRIP,MAINTAINING SAID WIPR IN BEARING RELATION AGAINST COATING MATERIAL FROMTHE STRIP EDGE AND PREVENT SUCH EXCESS MATERIAL FROM BEING CONTACTED BYTHE FLUID EMITTED FROM SAID FLUID NOZZLE.
 2. An improvemeNt according toclaim 1 in which the point of contact of said wiper is about 1 inchbeneath said point of impact of said fluid jet.
 3. An improvementaccording to claim 1 in which said strip is steel.
 4. An improvementaccording to claim 1 in which said coating material is zinc.